The invention relates to a filter block and to a filter body formed by the assembly of a plurality of filter blocks, used for the filtration of particles present in the exhaust gases of an internal combustion engine, particularly of the diesel type.
Conventionally, before being released to the open air, the exhaust gases may be purified by means of a particulate filter like the one shown in FIGS. 1 and 2, known in the prior art.
A particulate filter 1 is shown in FIG. 1 in a transverse cross section, along a cutting plane B-B shown in FIG. 2, and, in FIG. 2, in a longitudinal cross section along the cutting plane A-A shown in FIG. 1.
The particulate filter 1 conventionally comprises at least one filter body 3, inserted in a metal housing 5, intended to be traversed from an upstream side 7 to a downstream side 9, by the exhaust gases.
The filter body 3 generally comprises a plurality of filter blocks 11a-11i consisting of porous honeycomb structures, conventionally made from ceramic (cordierite, silicon carbide), assembled together by bonding by means of seals 12 of ceramic cement. The seals 12, substantially gastight, are conventionally about 1 mm thick. The ceramic cement generally consists of silica and/or silicon carbide and/or aluminum nitride.
The assembly thus formed can then be machined to the desired cross section, round or ovoid for example. The filter body 3 shown in FIGS. 1 and 2 has the shape of a cylinder with an axis C-C.
Among the filter blocks, a distinction is made between the internal filter blocks 11i and the external filter blocks 11a-11h. Unlike the internal filter blocks 11i, the external filter blocks 11a-11h are adjacent to the housing 5. Conventionally, a material 12′, gastight to the exhaust gases, is placed between the external filter blocks 11a-11h and the housing 5.
Conventionally, a filter block 11a-11i is substantially a rectangular parallelepiped, with length L, width l and height h. It comprises a plurality of straight, parallel channels 13, adjacent to one another, and with the same square cross section. The internal space 14 of a channel 13 is bounded by a side wall 17 terminating outwardly in an opening 19 and blocked, at the end opposite the opening 19, by a plug 20.
Two types of channels are distinguished. The inlet channels 13e comprise a plug 20e on the downstream side 9 and an opening 19e on the upstream side 7. The outlet channels 13s comprise a plug 20s on the upstream side 7 and an opening 19s on the downstream side 9.
The inlet channels 13e and outlet channels 13s are arranged alternately, in the width 1 and in the height h of the filter block.
As shown in FIG. 2, the exhaust gas stream F enters the filter body 3 via the openings 19e of the inlet channels 13e, passes through the side walls 17 of these channels to reach the outlet channels 13s, and escapes to the exterior via the openings 19s. The expression “filtration zone” of a side wall 17 is the area of this wall crossed by the exhaust gases.
Every passage of the gases through the filter block is accompanied by a pressure drop, mainly due to the passage of the gases through the filtration zones. It is desirable to minimize this pressure drop.
For the same gas flow rate entering the filter, the pressure drop is generally lower when the filtration zones of the channels are extended. In fact, a large filtration zone means, for an inlet channel, a large area available for the passage of the gases, and, for an outlet channel as well as an inlet channel, generally a large cross section limiting the pressure drop during the flow of the gases in the channel. However, a compromise is necessary to take account of the size, weight and cost requirements. To optimize this compromise, the filter is therefore adapted according to the pressure drop permissible in its application.
Patent application FR 0104686, filed by the Applicant, also proposes a filter body comprising a central part and a peripheral part having different channel densities and suitable for balancing the pressure drop at various points of the filter body.
Despite these improvements, a permanent need exists for solutions permitting a further reduction of the pressure drop across the filter while meeting the size requirements.
The object of the invention is to provide a filter block suitable for limiting the pressure drop resulting from the crossing of the filter by the gases.